Many hydraulic systems such as steering and brake systems have accumulators incorporated therein to provide a source of pressurized fluid to supplement the output of the pump when the pump is unable to supply the needed volume of fluid for operating the system. Such systems commonly have an accumulator charging valve (sometimes called unloader valves) to maintain the pressure level of the fluid within a predetermined range under normal operating conditions. The accumulator charging valves typically increase flow from the pump to the system when the pressure level drops below a predetermined low cut-in pressure level and reduces flow from the pump to the system when the pressure level reaches a predetermined high cut-out pressure level.
The commercially available accumulator charging valves typically have a differential area for regulating the cut-in and cut-out pressure levels. Those accumulator charging valves use poppet type flow regulation in which a conical shaped popper valve engages a valve seat. The design constraints of some hydraulic systems require a 90 percent area ratio such that the differential area is very small. One of the problems encountered therewith is that the contact areas are subjected to high stresses resulting in deformation of the contact area. Since the differential area is very small, such deformation changes the area ratio enough to change one or both of the cut-in and cut-out pressure levels.
Another type of accumulator charging valve is disclosed in U.S. Pat. No. 3,570,519. That accumulator charging valve is specifically described as being integrated into a common valve with a flow control and relief valve and appears to be limited to an open center circuit or a fixed displacement pump.
Thus, it would be desirable to have an accumulator charging valve which does not use poppet type flow regulation thereby eliminating the high stress contact area, is very stable when making the shift at the cut-in and cut-out settings and has independent adjustability for cut-in and cut-out pressure levels. Another desirable feature is that the accumulator charging valve be operational with a variable displacement pump such that the pump destrokes to a minimum displacement setting when the high pressure cut-out setting is reached and increases the displacement when the low pressure cut-in setting is achieved.